Hi, my name is Matthias Gritschneder, I am
a theoretical astrophysicist at the University Observatory of the
LMU Munich. My main interest is the interstellar medium (ISM) and
its transformation into stars. More specifically, I
employ hydrodynamical simulations to model the ISM in its gas phase. The ISM consists of a complex
network of dense and cold filamentary structures that are embedded in a diffuse, hotter medium. Filaments
are believed to form by compressional, converging flows as a result of interstellar turbulence, by
large scale spiral arms or by gravitational disk instabilities. It is generally believed that they are
destroyed as soon as they condense into stars which disperse their environment by their ionizing
radiation and stellar winds. After investigating these disruptive feedback processes early on
in my career, my focus has now shifted towards the formation of filaments and stars by an
external event. A possible event is the collision of a satellite with the disk of its host galaxy. In
addition, I work closely with observers on post-procession of simulations to enable a direct
comparison.

Projects

HII regions: During my PhD, I
showed in a series of publications that the pillars around
massive stars are filamentary structures which result from the
interaction of the local turbulent ISM with the ionizing flux of
a nearby star. After implementing simplified radiative transfer
into a SPH code, I picked a set of various initial conditions,
corresponding to subsets of the turbulent ISM (4pc×4pc) at
various densities and Mach numbers. When exposing these setups
to ionizing radiation, the ionization can penetrate much deeper
into the regions of lower density. These regions then exert
pressure on the adjacent cold gas, which is compressed into
pillar-like substructures and a morphology remarkably similar to
observed pillars evolves after t = 500kyr. At the tip of the
pillars, gravitational collapse occurs. To the left, the final
stage of the simulations is shown, the insert shows the
protostar forming inside the middle pillar.Core skills: Ionization implementation, Turbulence, SPH, Fortran,
IDL

Radiative Transfer: I worked with
Prof. Barbara Ercolano on the post-processing of simulations
described above to address the efficiency of the diffuse
ionization. We applied the Monte-Carlo code MOCASSIN to the final step of the main simulation
from my PhD. We then implemented a simplified prescription of
the diffuse radiation. The figure to the
right shows the final stage of the simulations, post-processed
as if observed in Hα. Core skills: Monte-Carlo radiative transfer, BPT-diagrams

Pipe Nebula: Another topic of my
studies has been the Pipe Nebula, which is located at the edge
of the Sco OB2 association. It is often considered to be the
ideal case of isolated star formation. Various models have
been proposed to explain the
formation and especially the observed core mass
function in the Pipe Nebula. Using analytical calculations, I
was able to show that the Pipe Nebula (shown to the left) can
be explained as an HII region shell swept up by θ
Ophiuchi (HD 157056, marked as the blue cross). The current size, mass,
and pressure of the region can be explained in this
scenario. ThePipe Nebula can best be described by a three
phase medium in pressure equilibrium. The pressure support is
provided by the ionized gas and is mediated by an atomic
component to confine the cores at the observed current pressure.Core skills: Radiative transfer (CLOUDY), databases (SIMBAD), IDL

Core Mass Function (CMF): In a
follow-up project on the local core mass function (CMF) with
Prof. Doug Lin and two graduate students Xu Huang (now Princeton) and
Tingtao Zhou (now MIT) we investigated what will happen if the
densest core in the Pipe collapses into a star. Assuming that
it is a massive star (M > 20Msun), it will ionize its
surrounding and thereby increase the temperature. As the cores
are embedded in the previously warm, now hot medium, the
change in pressure changes their boundary condition. This will
shift their characteristical Bonnor-Ebert mass. In turn, this
can explain the the observed (still debated) shift from the
core mass function (CMF) to the initial (stellar) mass
function (IMF). By invoking this mechanism, we are able to
show that the observed shift from the core mass function (CMF)
to the initial (stellar) mass function (IMF) can be
explained. This shift of about a factor of three is a subject of an ongoing debate. Core skills: Fragmentation, Coagulation, Student supervision

Solar System: In addition, I have
addressed the delivery of radioactive isotopes, especially 26Al into our Solar System. Analyzing simulations of the interaction of a supernova blast wave with a pre-stellar core, I was able to show that the gas can be sufficiently enriched by the supernova to explain the measured ratios in chondritic meteorites. To the left, I show the temperature in the early stages. The mixing of the supernova gas into the pre-existing core happens via instabilities in the shock front which lead to cold (blue) supernova gas. Core skills: Grid codes, Visualization (VISIT), Meteoritics}

Dwarf Galaxies: On a completely
different scale, I am very interested in the dark matter
content in dwarf galaxies, especially the `too big to fail'
problem. In a first attempt to solve this problem, I was able
to show with N-body simulations that the timely loss of
baryonic matter by e.g. a starburst with subsequent supernova
explosions can alleviate this problem. Core skills: N-Body simulations, ΛCDM

Oscillating Filaments: Recently,
I analyzed the stability of filaments in equilibrium between
gravity and internal as well as external pressure using the
grid based AMR-code RAMSES (Teyssier, 2002). Previous studies always investigated filaments parametrized as straight cylinders. However, if the cylinder is bent, e.g. with a slight sinusoidal perturbation, an otherwise stable configuration (f=0.5, where f=1 would collapse to a string) starts to oscillate, is triggered into fragmentation, and collapses. This previously unstudied behaviour allows a filament to fragment at any given scale, as long as it is bent. To the left I show the fragmentation for an initial sinusoidal perturbation with three periods.Core skills: AMR (Ramses), Visualization (Python)

Pillar formation and evolution:
Currently, I am involved (Co-I) in a series of observations of
pillars in Carina and other HII regions with the IFU
spectrograph MUSE (PI: Anna McLeod, McLeod et al. 2016). We
combined the results of the new Carina data set with archival MUSE data of a pillar in NGC 3603 and with previously published MUSE data of the Pillars of Creation in M16.
With a total of 10 analyzed pillars, we find tight correlations between the ionizing photon
flux and the electron density, the electron density and the distance from the ionizing
sources, and the ionizing photon flux and the mass-loss rate. The combined MUSE
data sets of pillars in regions with different physical conditions and stellar content
therefore yield an empirical quantification of the feedback effects of ionizing radiation.
In agreement with models, we find that dM/dt ~ Q1/2. For more details see ESO press release: https://www.eso.org/public/news/eso1639/Core skills: Observations, Photo-evaporation, Species abundances

b) My articles on arXiv:

Contact

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